Do Our Periods Sync Up? The Science Behind Menstrual Synchrony

All of sudden, the waste bins in dorms seem to fill instantaneously with tampon wrappers and tissue-wrapped pads. The mantra of every woman caught unprepared echoes through the halls, “Do you have a tampon?”

When women live together or spend a large amount of time together, do their periods sync up?

Any woman who’s ever gone to summer camp, lived in a dorm or had sisters would say, “Oh, yes they do.”

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A 1971 study of 135 women living together in a college dorm found that the longer the young women lived together, the closer their menstrual cycles became, reducing from an average of 6.4 days to 4.6 days.(1)

This group-bleeding became known as the McClintock effect, after the author of the study, Martha McClintock.

McClintock’s study has been widely held as the first example of pheromones — unconscious chemical signals — being shared among humans. (1)

A follow-up study in the 1990s that observed college roommates, athletes, lesbian couples, mothers, sisters, friends and even office mates found menstrual synchrony occurred at some times and not others, with no discernible reason.(1)

The problem lies in the varying lengths and duration of women’s menstrual cycles. The time between periods for different women can be anywhere from 21 to 35 days. Women normally menstruate for three to five days, but two to seven days is also normal for some women. (2)

“Given a cycle length of 28 days (not the rule—but an example), the maximum that two women can be out of phase is 14 days,” said Beverly I. Strassman.(3)

What appears to be synchronous ovulation and menstruation, is actually just the overlap of different cycles, a phasing in and out of synchrony.

So it’s not that we’re bleeding at exactly the same time, it’s just that each of us bleeds so often we appear to be hoeing the same row simultaneously.

A girl I live with (4 months) have very recently found that for the last two months, have synched almost perfectly with each other. Except now, even have started two weeks early and on the same day... Neither of us have been active, taking contraceptive or other medications nor have gained or lost an abnormal amount of weight. I think this would be hard evidence that synchronization is real... Now the question is which one of us got stressed enough to start this particular, two-week early fiasco, or was it both?

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